Patch for 3.4.x (bug #32747)

[Jordi Gutiérrez Hermoso]

Attached is a patch for #32747. I patched on the stable branch,
because that's where it should go. We really ought to merge stable
onto default. I tried to do it myself, but the two branches have
diverged considerably and I wasn't able to decide myself on how all
the merges should go. Particularly painful are the ChangeLogs, since
their structure is linear, but our VCS is not.

This was quite fun (no sarcasm). I had never gone quite so deep into
Octave code, and I had to dig around for quite some time with gdb
before I found the bug. It looks like a similar bug in changeset
11311:3bddc4ea8141, but the fix for this one was in a slightly
different direction. The Sparse<T>assign code could really use a lot
of cleanup, having copy-pasted code has resulted in patching almost
the same bug twice in each copy of the code, and it was quite
difficult to trace around with gdb. A single assign() call can result
into three levels deep of recursive calls on this function, which
seems a little messy to me.

By the way, who was responsible for 11311:3bddc4ea8141? The ChangeLog
and the commit message disagree.

- Jordi G. H.

squash-32747.patch
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